The main ingredient in any color enhancer is usually paprika. I buy paprika in bulk (I get it from the Amish, so it comes in a 10 lb bag) and feed one tablespoon a day. It's far cheaper than buying little containers of paprika every week or so, lol! In several months, you'll DEFINITELY see results! My mare is black, and although the paprika hasn't completely kept her from fading in the sun this summer, she is far darker than she was last summer without it.
I also feed two tablespoons of whole flax seed per day to keep their hair shiny. You can feed more if you wish. Some people feed a cup a day. I don't feed her ground flax seed, since it can spoil easily if not used all at once. Horses digest whole flax seed just as well.
When I took my mare to a trainer, to learn to drive, he took one look at her silky black coat and said, "Oh. She's been blanketed. Do we need to do that too?" It took a lot of convincing to get him to believe that she's never been blanketed! Too bad I don't have any good before and after pictures for you! Paprika and flax seed DO work though, and are a lot cheaper than buying a specific supplement. (And you'll notice that the SmartPak enhancer that a previous poster referred to contains- tada!- paprika.)
Paprika works to bring out the natural color of the coat, while flax seed keeps it smooth and shiny (or so I've found). It won't change your horse's color, just enhance it.